Many control units are increasing that electronically control in-vehicle devices mounted in a vehicle. This leads to requirements of decentralized or cooperative control by sharing of data between the control units. For instance, the following is studied that control units are classified into groups such as a body group, a power-train group, or an information group, so each group of control units is networked as a network. The networked groups (or networks) are connected via gateway units that are capable of repeating data between the networks. The body group includes seats, doors, or the like. The power-train group includes an engine and an electronic throttle. The information group includes a navigation device, a VICS (Vehicle Information and Communication System), and an ETC (Electronic Toll Collection system).
In general, a control unit has a high power consumption mode and a low power consumption mode. When an ignition switch is turned off and a vehicle is being parked, a basic function as a vehicle is unnecessary. Therefore, the mode of the control unit is switched to the low power consumption mode. The low power consumption mode enables power consumption of each control unit to decrease, for instance, by the following operations: decreasing an operating frequency from that of a usual operating mode, continuing stopping until a given trigger signal is inputted, or the like.
Suppose that control units are grouped into a network. Here, when one control unit is in a high power consumption mode, the other control units are in concert set to the high power consumption mode for cooperative operations between the control units within the network. Further suppose that control units are grouped into multiple networks. Here, a gateway unit may manage operating modes for relevant control units within the multiple networks. In sum, suppose that certain control units that need to mutually exchange data are located in different relevant networks and the gateway unit receives information that one control unit within the certain control units is in the high power consumption mode. In this case, the gateway unit conducts a mode management for the relevant networks where the certain control units belong, to cause the relevant networks to work in the high power consumption mode. Here, the gateway unit usually manages an operating mode with respect to a network instead of individual control units in order to avoid complexity.
Such the gateway unit can collectively manages an operating mode for each of the networks when only one gateway unit is included in a communications system. Here, transferring between a low power consumption mode and a high power consumption mode can be thereby conducted smoothly.
The number of networked control units or the number of communicated data continues to increase, so a size of each network increases. In this case, only one gateway unit may not manage the large network. Data communications speeds between control units tend to limit the number of control units connected with one network. As the number of networks increases, it becomes difficult that only one gateway unit controls communications between the multiple networks. More than one gateway unit is supposed to be adopted in the communications system including the multiple networks.
In this case, a certain network connects with multiple networks via multiple gateway units, which has a potential of causing troubles in a mode management. When a gateway unit receives data of being in a high power consumption mode from one control unit, the gateway unit sends this data to all networks connected with the gateway unit. Thus, the gateway unit manages, in the high power consumption mode, the operating mode for all the networks connected. When multiple gateway units are included in this communications system, data of being in the high power consumption mode may be mutually exchanged between these multiple gateway units. Consequently, each control unit may be unable to change to a low power consumption mode.